This invention relates to flexible polyurethane foams used in furniture and seat cushions, and as components of bedding mattresses and mattress pads. Produced under vacuum conditions from certain foaming mixtures, the foams of this invention have higher support factors at lower densities than more conventional polyurethane foams previously used in these applications.
Cellular polyurethane structures typically are prepared by generating a gas during polymerization of a liquid reaction mixture comprised of a polyester or polyether polyol, a polyisocyanate, a surfactant, catalyst and one or more blowing agents. The gas causes foaming of the reaction mixture to form the cellular structure. The surfactant stabilizes the structure.
Polyurethane foams with varying density and hardness may be formed. Hardness is typically measured as IFD (xe2x80x9cindentation force deflectionxe2x80x9d) or CFD (xe2x80x9ccompression force deflectionxe2x80x9d). Specifically, IFD25 is the force required to compress the foam to 25% of its original thickness or height, and IFD65 is the force required to compress the foam to 65% of its original thickness or height. Tensile strength, tear strength, compression set, air permeability, fatigue resistance, support factor, and energy absorbing characteristics may also be varied, as can many other properties. Specific foam characteristics depend upon the selection of the starting materials, the foaming process and conditions, and sometimes on the subsequent processing. Among other things, polyurethane foams are widely used for cushioning applications, such as seat cushions, furniture cushions, pillows and mattress pads, and as components of mattresses.
In many cushioning applications, the cushioning effectiveness is measured by the support factor, which is the ratio of IFD65 to IFD25. A high support factor value is generally associated with higher cushioning comfort. Physically, a high IFD65 value is preferred. A low IFD65 value indicates that the foam may xe2x80x9cbottom outxe2x80x9d without offering very much cushioning support. A lower IFD25 value is preferred because a foam with a lower IFD25 value usually has a softer initial feel, which is perceived as providing more cushioning comfort. A higher support factor generally results for foams with the combination of higher cushioning support and better cushioning comfort.
Once the foam-forming ingredients are mixed together, it is known that the foam may be formed under either elevated or reduced controlled pressure conditions. PCT Published Patent Application WO 93/09934 discloses methods for continuously producing slabs of urethane polymers under controlled pressure conditions. The foam-forming mixture of polyol, polyisocyanate, blowing agent and other additives is introduced continuously onto a moving conveyor in an enclosure with two sub-chambers. The foaming takes place at controlled pressure. Reaction gases are exhausted from the enclosure as necessary to maintain the desired operating pressure. The two sub-chambers, a saw, and air tight doors are operated in a manner that allows for continuous production of slabstock polyurethane foam.
U.S. Pat. No. 5,804,113 to Blackwell, et al., shows a method and apparatus for continuously producing slabstock polyurethane foam under controlled pressure conditions in which a layer of gas surrounds the reaction mixture during free rise expansion of the reaction mixture to prevent pressure fluctuations. Blackwell generally describes foam reaction mixtures that may include a variety of polyols and polyisocyanates, and does not express preference for any specific combinations. The working examples use only conventional TDI.
U.S. Pat. No. 4,777,186 to Stang, et al., describes a method of foaming in a pressurized chamber held above atmospheric pressure (i.e., in the range of about 0.5 to 1000 psig). In addition to the gases emitted during foaming, additional gases may be introduced into the chamber to maintain the elevated pressure during foaming. The resulting foams have a higher ILD to density ratio than those previously known in the art.
U.S. Pat. No. 5,698,609 to Lockwood, et al. discloses energy absorbing foams intended for use in shipping and packaging containers. The open cell polyurethane foams are prepared from a combination of specific polyols reacted with diphenylmethane diisocyanate (MDI) and polyphenylmethylene diisocyanate (poly-MDI) at atmospheric pressure. The resulting foams have a higher density from 1.5 to 5 pounds per cubic foot, preferably 2.0 to 3.0 pounds per cubic foot, and an air flow of 0.05 to 0.5 scfm.
Flexible polyurethane foams with high densities in the range of 35 to 70 kg/m3 (or 2.2 to 4.4 lb/ft3) are produced by the method disclosed in U.S. Pat. No. 5,194,453 to Jourquin, et al. Polyether polyols with molecular weights in the range of 1400 to 1800 and having primary hydroxyl group content over 50% are reacted with organic polyisocyanates that may be TDI, MDI or mixtures of TDI with MDI. The foams may be produced by frothing the reaction mixture, or alternatively, under vacuum conditions. Support factor was not reported, although deformation tests were conducted and the foams are indicated to have improved comfort properties.
Higher density polyurethane foams (30 kg/m3 or about 1.9 lb/ft3) are produced with the polyol combinations disclosed in U.S. Pat. No. 5,668,378 to Treboux, et al. The foam-forming mixture includes 80 to 99.8 percent by weight of a high functionality polyol or polyol blend with 8 to 25% EO, functionality from 3.2 to 6.0 and an equivalent weight of 1000 to 4000, a minor portion of a graft polyol, and an organic polyisocyanate that preferably is a mixture of TDI. The foams are foamed at atmospheric pressure.
Foams with lower density, but still excellent support characteristics are continually sought for furniture, mattress components and pillows. Such lower density foams make it possible to reduce the total weight without compromising comfort. The prior art does not disclose methods for making high resilience polyurethane foams with lower densities yet higher support factors.
According to the invention, flexible, high resilience, polyurethane cushioning foams with higher support factors at lower or equivalent densities are produced using a method comprising preparing a foam reaction mixture and foaming that mixture at vacuum conditions, preferably at pressures in the range of 0.5 to 0.9 bar (absolute), most preferably 0.6 to 0.8 bar (absolute). The reaction mixture contains (a) a polyol mixture of (i) about 40 to 90 percent by weight total polyols of a polyether polyol having a primary hydroxyl group content of greater than 75 percent and from 14 to 30 percent ethylene oxide groups, and having a hydroxyl number in the range of about 20 to 36 and a functionality from 2.8 to 3.5, and (ii) about 10 to 60 percent by weight total polyols of a graft polyol having a ratio of styrene to acrylonitrile of about 70 to 30, and having a hydroxyl number in the range of about 25 to 50 and a functionality from 2.5 to 3.0; (b) an organic polyisocyanate selected from the group consisting of methylene diisocyanate and methylene diisocyanate mixed with toluene diisocyanate, wherein if a mixture of methylene diisocyanate and toluene diisocyanate is used, the polyisocyanate mixture comprises from about 5 to 20 percent by weight toluene diisocyanate and about 80 to 95 percent by weight methylene diisocyanate, wherein at least 50 percent of the methylene diisocyanate is 4,4xe2x80x2 methylene diisocyanate, and wherein the isocyanate index is in the range of 95 to 110; and (c) from about 2 to 3.5 parts per hundred parts polyol of water as a blowing agent.
Most preferably, the foam-forming composition contains up to 2 parts per hundred parts polyol of an amine catalyst, up to 2 parts per hundred parts polyol of a surfactant, up to 0.5 parts per hundred parts polyol of an organotin catalyst, and up to 2 to 6 parts per hundred parts polyol of a cross linking agent.
In addition, excellent results have been obtained using a polyol combination of (a) from 50 to 70% by weight total polyols of polyether polyol (functionality 3.1 to 3.3), having a primary hydroxyl group content of greater than 75 percent and from 14 to 20 percent EO groups and a hydroxyl number in the range of 28 to 32, and (b) about 30 to 50 percent by weight total polyols of a graft polyol having a ratio of styrene to acrylonitrile of about 70 to 30, and having a hydroxyl number in the range of about 25 to 30 and a functionality from 2.6 to 2.9. Moreover, the polyisocyanate mixture, if a mixture is used, most preferably contains no more than 10% by weight toluene diisocyanate and at least 55 % by weight 4,4xe2x80x2 methylene diisocyanate, with an isocyanate index in the range of 100 to 105. In this preferred embodiment, from about 2.8 to 3.2 parts per hundred parts polyol of water as a blowing agent; and up to 1.0 parts per hundred parts polyol of a surfactant are included in the reaction mixture.
The resulting lower density, high resilience polyurethane foams have densities in the range of 1.4 to 1.8 pounds per cubic foot, and a support factor (SAG) above about 2.4, preferably above about 2.5.